Growing concerns about the environment has resulted in development of new environmentally friendly technologies, new materials, and new ways to reduce and minimize wastes [Manahan, S. E. Environmental Chemistry, 6th ed., CRC Press: Boca Raton, Fla., 1994]. One of the wastes produced by contemporary society in abundant quantity is municipal sewage sludge, euphemistically often referred to as biosolids. Biosolids are a mixture of exhausted biomass generated in the aerobic and anaerobic digestion of the organic constituents of municipal sewage along with inorganic materials such as sand and metal oxides. According to the United States Environmental Protection Agency (EPA), 6.9 million tons of biosolids (dry basis) were generated in 1998 and only 60% were used beneficially [Biosolid Generation, Use, and Disposal in The United States: EPA530-R-99-009, September 1999; www.epa.gov]. The EPA report estimates an annual 2% increase in the quantity of biosolids produced.
The abundance of raw sewage sludge produces one of the major environmental problems of contemporary civilization. Various methods have been proposed for its disposal [Manahan S. E. Environmental Chemistry, 6th ed., CRC Press: Boca Raton, Fla., 1994]. Ocean dumping was popular until recently, however is no longer an option because of stricter environmental regulations [Biosolid Generation, Use, and Disposal in The United States: EPA530-R-99-009, September 1999; www.epa.gov]. Among the most often used methods of disposal are landfilling, cropland application, and incineration [Manahan S. E. Environmental Chemistry, 6th ed.; CRC Press: Boca Raton, Fla., 1994.]. Other methods that have been used to dispose of or utilize municipal sewage sludge [Biosolid Generation, Use, and Disposal in The United States: EPA530-R-99-009, September 1999; www.epa.gov], include road surfacing, conversion to fertilizer, compression into building blocks, and carbonization [Manahan, S. E. Environmental Chemistry, 6th ed., CRC Press: Boca Raton, Fla., 1994; Biosolid Generation, Use, and Disposal in The United States: EPA530-R-99-009, September 1999; www.epa.gov; Sutherland, J. U.S. Pat. No. 3,998,757 (1976); Nickerson, R. D.; Messman, H. C., U.S. Pat. No. 3,887,461 (1975)]. Specifically, the residue of incineration can be used in construction materials or road surfacing.
Although incineration is effective in reducing the volume of sludge and produces useful end products, cleaning of the flue gases generated requires effective and expensive scrubbers. The application of raw sewage sludge as a fertilizer produces odor problems and is also associated with the risk of contamination of the soil by heavy metals and pathogens. A more efficaceous and safer alternative is the pyrolytic carbonization of sludge to obtain useful sorbents [Piskorz J, Scott D S, Westerberg, I B. Flash pyrolysis of sewage sludge, Ind. Proc. Des. Dev. 1996; 25: 265-270; Chiang, P C., You, J H. Use of sewage sludge for manufacturing adsorbents, Can. J. Chem. Eng. 1987; 65: 922-927; Lu, G Q, Low J C F, Liu C Y, Lau A C. Surface area development of sewage sludge during pyrolysis, Fuel 1995; 74: 3444-3448; Lu G Q, Lau D D. Characterization of sewage sludge-derived adsorbents for H2S removal. Part 2: surface and pore structural evolution in chemical activation. Gas Sep. Purif. 1996; 10: 103-111; Lewis F M. Method of pyrolyzing sewage sludge to produce activated carbon, U.S. Pat. No. 4,122,036 (1977)].
Since 1976, several patents have been issued on carbonization of sewage sludge and various applications of the final materials [Nickerson, R. D.; Messman, H. C., U.S. Pat. No. 3,887,461(1975); Lewis, F. M. U.S. Pat. No. 4,122,036 (1977); Kemmer, F. N.; Robertson, R. S.; Mattix, R. D. U.S. Pat. No. 3,619,420 (1971)]. The carbonization of sludge was first patented by Hercules, Inc. [Sutherland, J. Preparation of activated carbonaceous material from sewage sludge and sulfuric acid. U.S. Pat. No. 3,998,757 (1976)]. The process was further investigated by Chiang and You [Chiang, P C., You, J H. Use of sewage sludge for manufacturing adsorbents, Can. J. Chem. Eng. 1987; 65: 922-927] and Lu, et al. [Lu, G Q, Low J C F, Liu C Y, Lau A C. Surface area development of sewage sludge during pyrolysis, Fuel 1995; 74: 3444-3448.; Lu G Q, Lau D D. Characterization of sewage sludge-derived adsorbents for H2S removal. Part 2: surface and pore structural evolution in chemical activation. Gas Sep. Purif. 1996; 10: 103-111]. Both simple pyrolysis and pyrolysis after addition of chemical activation agents such as zinc chloride or sulfuric acid were used. Carbonization of sludge in the presence of chemical activating agents such as zinc chloride and sulfuric acid produces new sorbents, with patented applications in such processes as removal of organics in the final stages of water cleaning [Lewis, F. M. U.S. Pat. No. 4,122,036 (1977)] and removal of chlorinated organics [Kemmer, F. N.; Robertson, R. S.; Mattix, R. D. U.S. Pat. No. 3,619,420 (1971)].
The process of carbonization of biosolids has been studied in detail using different chemical agents and various conditions [Chiang, P. C.; You, J. H. Can. J. Chem. Eng. 1987, 65, 922; Lu, G. Q; Low J. C. F.; Liu, C. Y.; Lau A. C. Fuel 1995, 74, 3444; Lu, G. Q.; Lau, D. D. Gas Sep. Purif. 1996, 10, 103; Lu, G. Q. Environ. Tech. 1995, 16, 495]. The sorbents obtained had relatively high surface area (100-200 m2/g for physical activation and up to 400 m2/g for chemical activation) and developed microporosity. As suggested by Chiang and You, the high content of inorganic matter, usually around 75%, together with the microporosity promotes the adsorption of organic species such as methyl ethyl ketone or toluene [Chiang, P C., You, J H. Use of sewage sludge for manufacturing adsorbents, Can. J. Chem. Eng. 1987; 65: 922-927]. In general, materials obtained as a result of the treatment have surface areas between 100 and 500 m2/g, but their performance as adsorbents has been demonstrated to be much worse than that of activated carbons. The ability of these adsorbents to remove organics such as phenols, or sulfur dioxide and hydrogen sulfide [Lu, G. Q.; Lau, D. D. Gas Sep. Purif. 1996, 10, 103; Lu, G. Q. Environ. Tech. 1995, 16, 495] have been tested so far; their capacity for the adsorption of SO2 reported by Lu was less than 10% of the capacity of Ajax activated carbon [Lu, G. Q. Environ. Tech. 1995, 16, 495]. Lu and coworkers used the sorbents obtained from sludge by chemical activation as media for the removal of hydrogen sulfide [Lu G Q, Lau D D. Characterization of sewage sludge-derived adsorbents for H2S removal. Part 2: surface and pore structural evolution in chemical activation. Gas Sep. Purif. 1996; 10: 103-111]. Their removal capacity was only 25% of that of Calgon activated carbons and the mechanism and efficiency of the process were not studied in detail.
Since hydrogen sulfide is the main source of odor from sewage treatment plants the possibility of using sewage sludge as a source of adsorbents for H2S is appealing. The idea is even more attractive when the mechanism of adsorption of hydrogen sulfide is taken into account. As proposed elsewhere [Hedden K, Huber L, Rao B R. Adsorptive Reinigung von Schwefelwasserstoffhaltigen Abgasen VDI Bericht 1976;37: 253; Adib F, Bagreev A, Bandosz T J. Effect of surface characteristics of wood based activated carbons on removal of hydrogen sulfide. J. Coll. Interface Sci. 1999; 214: 407-415; Adib F, Bagreev A, Bandosz
TJ. Effect of pH and surface chemistry on the mechanism of H2S removal by activated carbons. J. Coll. Interface Sci. 1999; 216: 360-369] H2S is first adsorbed in the water film present on the carbon surface, followed by dissociation and adsorption of HS− in the micropores. In the next step, HS− is oxidized to various sulfur species. The speciation of the final products of oxidation depends on the pH of the activated carbon surface [Adib F, Bagreev A, Bandosz T J. Effect of pH and surface chemistry on the mechanism of H2S removal by activated carbons. J. Coll. Interface Sci. 1999; 216: 360-369; Adib F, Bagreev A, Bandosz T J. Analysis of the relationship between H2S removal capacity and surface properties of unimpregnated activated carbons. Environ. Sci. Technol. 2000; 34: 686-692; Adib F, Bagreev A, Bandosz T J. Adsorption/oxidation of hydrogen sulfide on nitrogen modified activated carbons. Langmuir 2000; 16: 1980-1986]. This mechanism is based on the study on unmodified carbons [Adib F, Bagreev A, Bandosz T J. Effect of surface characteristics of wood based activated carbons on removal of hydrogen sulfide. J. Coll. Interface Sci. 1999; 214: 407-415; Adib F, Bagreev A, Bandosz T J. Effect of pH and surface chemistry on the mechanism of H2S removal by activated carbons. J. Coll. Interface Sci. 1999; 216: 360-369; Adib F, Bagreev A, Bandosz T J. Analysis of the relationship between H2S removal capacity and surface properties of unimpregnated activated carbons. Environ. Sci. Technol. 2000; 34: 686-692]. In the case of catalytic carbons containing nitrogen it was proposed that nitrogen-containing basic centers located in the micropores are the high energy adsorption sites playing an important role in the oxidation of hydrogen sulfide to sulfuric acid [Adib F, Bagreev A, Bandosz T J. Adsorption/oxidation of hydrogen sulfide on nitrogen modified activated carbons. Langmuir 2000; 16: 1980-1986.]. The latter as the final product makes the regeneration feasible using simple methods such as washing with water [Adib F, Bagreev A, Bandosz T J. On the possibility of water regeneration of impregnated activated carbons used as hydrogen sulfide adsorbents, Ind. Eng. Chem. Res. 2000; 39: 2439-2446; Bagreev A, Rahman H, Bandosz T J. Study of H2S adsorption and water regeneration of spent coconut-based activated carbon. Environ. Sci. Technol. 2000; 34: 4587-4592]. In the case of catalytic carbons such as Centaur® the basic centers are introduced using the special urea modification process [Matviya T M, Hayden R A. Catalytic Carbon. U.S. Pat. No. 5,356,849 (1994)]. Since sewage sludge contains a considerable amount of organic nitrogen, carbonization of such species can lead to the creation of basic nitrogen groups within the carbon matrix which again have been proven to be important in the oxidation of H2S [Adib F, Bagreev A, Bandosz T J. Adsorption/oxidation of hydrogen sulfide on nitrogen modified activated carbons. Langmuir 2000; 16: 1980-1986; Matviya T M, Hayden R A. Catalytic Carbon. U.S. Pat. No. 5,356,849 (1994)]. Another advantage to the use of sludge as a starting material is the presence of significant amounts of iron added to the raw sludge as a dewatering conditioner; iron is also considered to be a catalyst for H2S oxidation [Katoh H., Kuniyoshi I., Hirai M., Shoda M. Studies of the oxidation mechanism of sulfur containing gases on wet activated carbon fibre. Appl. Cat. B: Environ. 1995;6: 255-262; Stejns M, Mars P. Catalytic oxidation of hydrogen sulphide. Influence of pore structure and chemical composition of various porous substances. Ind. Eng. Chem. Prod. Res. Dev. 1977; 16: 35-41; Cariaso, O. C. and Walker P L. Oxidation of hydrogen sulphide over microporous carbons. Carbon 1975; 13: 233-239].
Primarily caustic-impregnated carbons have been used as adsorbents of hydrogen sulfide at sewage treatment plants. Because of the presence of KOH or NaOH their pH is high, which ensures that hydrogen sulfide is oxidized to elemental sulfur. The process is fast and caustic impregnated carbons have high hydrogen sulfide breakthrough capacity. Such materials have a H2S breakthrough capacity measured using accelerated test (not suitable for virgin carbons and other adsorbents), which should be around 140 mg/g. In one example of its use, the New York City Department of Environmental Protection installed 118 carbon vessels in 12 sewage treatment plants. Each vessel contains about 10 tons of activated carbon adsorbent.
Caustic-impregnated carbons, although efficient for H2S removal, have many disadvantages which recently have attracted the attention of researchers toward alternative sorbents, unmodified activated carbons. The disadvantages of caustic-impregnated carbons are as follows:    1) Limited capacity for physical adsorption of VOCs (volatile organic compounds) due to the presence of caustic materials in the carbon pore system.    2) Low self-ignition temperature, which may result in fire inside the carbon vessel.    3) Special safety precautions in dealing with caustic materials have to be applied.    4) High density because of the presence of water.    5) Higher cost than that of unmodified carbons.
The results of recent studies have shown that at very low concentrations of hydrogen sulfide (as is present at sewage treatment plants), unmodified carbons can work effectively as adsorption/oxidation media. Thus, there is a great interest in the development of new types of adsorbents for use in sewage treatment facilities.